The non-invasive evaluation of intracranial pressure may provide important medical information when evaluating head injuries (e.g. concussions) or effects from other medical conditions (e.g. strokes, brain tumors, and meningitis). Prior methods of this analysis relied on pulsed phrase-locked loop technology (“PPLL”), and relied on a quadrature phase detector to detect phase changes between a reference oscillator path and a second path resulting from a wave passing though the cranial vault, and then reestablish quadrature between the two paths. Whenever the second path changed in any way, these instruments adjusted the reference oscillator frequency until quadrature was re-established. The PPLL instrument would then record this change in frequency and use it to estimate the phase change that occurred in the second path. These instrument's measurement of frequency, f, is related to the ultrasonic wave speed v and path length l by Equation 1:
            Δ      ⁢                          ⁢      f        f    =                    Δ        ⁢                                  ⁢        v            v        -                  Δ        ⁢                                  ⁢        l            l      
These methods, however, suffer from drawbacks, including problems with accuracy, precision, noise level and stability. For example, other elements in the measurement paths, such as coaxial cables, amplifiers, signal splitters, ultrasonic transducers and so on, also affect a signal's phase as the frequency changes. This creates errors as well as uncertainties, as the PPLL systems operate under that assumption that the quadrature phase detectors respond only to phase changes in the signal path relative to the reference oscillator path. What's more, reflections from the skull also shift phase due to frequency changes, and therefore also create unintended and unaccounted phase-shift contributions to the systems output. Another drawback is that the quadrature phase detector prevents the measurement technology from measuring phase differences linearly, and therefore cannot provide numerical output in phase.